[flang-rt] Set supported_sources when FLANG_RT_EXPERIMENTAL_OFFLOAD_SUPPORT=CUDA (#156078)

When building for the CUDA support sources should be set to the gpu
sources.

Author: clementval

flang-rt/lib/runtime/CMakeLists.txt

@@ -177,6 +177,8 @@ endif ()
 
 if ("${LLVM_RUNTIMES_TARGET}" MATCHES "^amdgcn|^nvptx")
   set(sources ${gpu_sources})
+elseif(FLANG_RT_EXPERIMENTAL_OFFLOAD_SUPPORT STREQUAL "CUDA")
+  set(sources ${supported_sources})
 else ()
   set(sources ${supported_sources} ${host_sources} ${f128_sources})
 endif ()